This invention relates to a linear hydraulic motor having a frangible piston head.
A typical linear hydraulic motor includes a cylinder, a rod in the cylinder and a piston attached to the rod including a seal dividing the cylinder into two chambers such that hydraulic fluid may be ported to one chamber or the other to cause the piston to move in a desired direction. In some applications a plurality of such motors may be connected to a particular driven member such as a control surface to provide redundant control. Where such motors may be damaged by ballistic action, one motor may be disabled and another can continue to effect the desired control function so long as the rod in the disabled motor is free to move. It frequently happens, however, that the cylinder wall of the disabled motor is breached in such a way as to cause metal to be deformed into the path of the piston, effectively causing it to be blocked and preventing the viable cylinder from providing the desired control
The above problem has been dealt with by constructing the piston in such a way that portions of it can break away when driven into an inwardly projecting metal obstacle. In one prior art design, the piston and rod are made of hardened steel. To provide the desired weakness in the piston, the seal groove is made wider than usual resulting in narrower than usual flanges on the sides of the packing, which flanges are then sawed through in an axial direction to approximately the bottom of the seal groove with a large number of cuts around the periphery of the piston. The resulting piston and rod are then heat treated to enhance the brittleness of the flanges. This arrangement has proved to be less than totally satisfactory because the heat treating tends to cause hydrogen embrittlement of the entire rod and piston assembly which has resulted in premature fracturing of the rod. Should the heat treating be insufficient, the flange sectors may be too strong to fracture as desired. Also, the use of the extra wide seal groove and packing has resulted in a higher incidence and amount of hydraulic fluid leakage than is desirable.